Beam admission devices are used, for example, in x-ray machines such as x-ray computed tomography devices for admitting the x-ray radiation emanating from an x-ray source onto a desired examination or detection region, for example in the form of a fan. By way of the admission unit, inter alia, direct irradiation outside of the desired examination region by the x-ray radiation can be avoided, at least to the greatest extent. The latter in particular decreases the radiation dose applied during the examination of a body.
It is known to use a slit screen to admit the x-ray radiation; the former comprises a planar tantalum plate with admission slits provided therein. By using admission slits having different widths, it is possible to admit the x-ray radiation emanating from an x-ray source onto examination or detection regions having different widths, which is desirable in the case of x-ray computed tomography using multirow detectors, for example. It is possible to obtain different admissions by longitudinal displacement of the tantalum plate, that is to say a displacement perpendicular to the longitudinal direction of the admission slits, which positions a respectively suitable admission slit on the beam port of the x-ray source.
However, it is disadvantageous in this case that the length of the tantalum plate rapidly increases with an increasing number of desired admission regions and hence increasing number of admission slits. Occasionally, this is due to the fact that, depending on the geometric conditions, a prescribed minimum distance has to be observed between adjacent admission slits. The minimum distance depends on, inter alia, the distance of the tantalum plate from the beam port and the aperture of the beam port. The space available for attaching and to-and-fro displacement of the tantalum plate is substantially limited by housing walls and the like, for example by the housing of the gantry of an x-ray computed tomography scanner, which should be as thin as possible for patient-psychological reasons; hence the number of admission possibilities that can be implemented is limited.
In addition, the complexity involved for the mechanical support of the tantalum plate increases with increasing linear dimensions. This is all the more important in x-ray computed tomography since non-negligible acceleration forces act on the tantalum plate during circular or helical scans of the body.
Furthermore, conventional drive systems for moving the tantalum plate to-and-fro, which for example comprise a motor-driven threaded spindle for moving the tantalum plate, have a low dynamic range. The latter means that, for example, setting two admission possibilities based on two admission slits which are at a distance from one another requires a relatively long time; this is in contrast to a time-optimized examination of a body, in particular using different recording modes. In addition, a drive system with a threaded spindle is not very stiff and has relatively high wear and tear. However, high wear and tear in turn leads to the play in the drive unit rapidly increasing, which reduces in a non-negligible way the achievable positional accuracy of the admission slits.
It is known to design the tantalum plate in an arced shape, in particular in order to decrease its linear extent. However, in this case too, the space available in the cramped spatial conditions is quickly exhausted with increasing numbers of admission slits, in particular with respect to the minimum distance of the admission slits.
For example, so-called jaw screens provide a space-saving implementation of a multiplicity of admission possibilities. These screens can for example comprise two jaws which can be moved against one another, or two eccentrically mounted rollers which can be rotated. Moving the jaws or rotating the rollers affords the possibility of realizing many admission slits with different widths. However, a disadvantage of the jaw screen is that two jaws or rollers always have to be synchronously moved in order to set a desired width of an admission slit. This makes the design, and the required control and drive devices for setting the admission slit, more complicated and more expensive.